At present, in cutting edge research fields, for the purpose of identification of vital functions or behavioral analysis/interaction of proteins, various techniques have been developed to observe living cells over a period from a few days to several weeks. As a microscope observation technique for observing lesions within living cells, wide use has been made of a fluorescence observation technique. In this fluorescence observation technique, after biological samples (e.g., living cells) have been dyed using a fluorescent material such as a specific fluorescent protein as a luminescent marker, excitation light is irradiated onto the biological samples so that fluorescence is generated. The presence and locations of specific sites within the biological samples, such as lesions, are then detected by observing the sample.
An objective lens of a biological microscope is generally configured to observe an object under observation (i.e., a specimen) through a cover glass. It is assumed that the thickness of the cover glass is a constant standard value, and the aberrations that are generated by the cover glass are designed to be favorably corrected by the objective lens. However, production flaws are inherent in making cover glasses. Further, depending upon the particular observation technique used, there are occasions during which an object under observation is observed through a flat plate having surfaces that are roughly parallel, such as a cover glass or a petri dish, but with the thickness of the flat plate being different from the standard value for which the objective lens was designed.
Consequently, when the thickness of a cover glass is different from the standard value, when the thickness fluctuates due to manufacturing tolerances, or when a combination of these two conditions occurs, the aberrations generated by the cover glass will not be sufficiently corrected by the objective lens, and thus the image quality deteriorates. More specifically, the higher the numerical aperture (hereinafter NA) of the objective lens of the microscope, the more apparent the deterioration of image quality becomes.
Japanese Laid Open Patent Application H03-58492 and Japanese Publication 3371934 disclose conventional microscope objective lenses wherein aberrations generated by a cover glass, that is arranged between the plane of an object under observation and the objective lens, are corrected either for the cover glass being of a non-standard thickness or for having a thickness that fluctuates due to manufacturing tolerances.